Sheep Casings, which is right for me?

Looking at sheep casings for making sausages. I especially like making dried and fermented sausages like salami or peperoni in thin casings. But when it comes to buy some more casings, suddenly, there are so many different types.

What does it mean if a casing is 20/22 or 22/24 or 24/26. Which is smaller? Also what's the difference between "pork sausage quality sheep casings" and "wiener quality sheep casings"?

What I am looking for is the smallest and strongest (easiest to work with), which should I get?

The numbers refer to diameter, in millimeters, so you'll want the smallest. Don't know about the difference between "pork sausage" versus "wiener" casings, but my guess is the wiener casings are the larger, versus the smaller sheep casings typically used for (smaller) breakfast sausages.